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have two people for primary mirror collimation, one to look in
the focuser while the other adjusts the collimation screws.)
The view down the focuser should now resemble Figure 8a.
The secondary mirror is centered under the focuser; the
reflection of the primary mirror is centered in the secondary
mirror, and the reflection of the secondary mirror is centered
in the reflection of the primary mirror.
A simple star test will tell you whether the optics are accu-
rately collimated.
Star-Testing Your Telescope
When it is dark, point the telescope at a bright star and center it
in the eyepiece's field of view with the R.A. and Dec. slow-motion
controls. Slowly rack the image out of focus with the focusing
knob. If the telescope is correctly collimated, the expanding disk
should be a circle. If it is unsymmetrical, the scope is out of colli-
mation. The dark shadow cast by the secondary mirror should
appear in the very center of the out-of-focus circle, like the hole
in a doughnut. If the "hole" appears off-center, the telescope is
out of collimation (Figure 12).
Out of collimation
Figure 12. A star test will determine if a telescope's optics are properly
collimated. An unfocused view of a bright star through the eyepiece
should appear as illustrated on right if optics are perfectly collimated. If
circle is unsymmetrical, as in illustration on left, scope needs collimation.
If you try the star test and the bright star you have selected is
not accurately centered in the eyepiece, then the optics will
always appear out of collimation, even though they may be
perfectly aligned. It is critical to keep the star centered, so over
time you will need to make slight corrections to the telescope's
position in order to account for the sky's apparent motion.
7. Using Your Telescope—
Astronomical Observing
Choosing an Observing Site
When selecting a location for observing, get as far away as
possible from direct artificial light such as street lights, porch
lights, and automobile headlights. The glare from these lights
will greatly impair your dark-adapted night vision. Set up on a
grass or dirt surface, not asphalt, because asphalt radiates
more heat. Heat disturbs the surrounding air and degrades
the images seen through the telescope. Avoid viewing over
rooftops and chimneys, as they often have warm air currents
rising from them. Similarly, avoid observing from indoors
12
through an open (or closed) window, because the tempera-
ture difference between the indoor and outdoor air will cause
image blurring and distortion.
If at all possible, escape the light-polluted city sky and head
for darker country skies. You'll be amazed at how many more
stars and deep-sky objects are visible in a dark sky!
"Seeing" and Transparency
Atmospheric conditions vary significantly from night to night.
"Seeing" refers to the steadiness of the Earth's atmosphere
at a given time. In conditions of poor seeing, atmospheric tur-
bulence causes objects viewed through the telescope to
"boil". If, when you look up at the sky with just your eyes, the
stars are twinkling noticeably, the seeing is bad and you will
be limited to viewing with low powers (bad seeing affects
images at high powers more severely). Planetary observing
may also be poor.
In conditions of good seeing, star twinkling is minimal and
images appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
after midnight, when much of the heat absorbed by the Earth
during the day has radiated off into space.
Especially important for observing faint objects is good "trans-
parency" - air free of moisture, smoke, and dust. All tend to
scatter light, which reduces an object's brightness. Transparency
is judged by the magnitude of the faintest stars you can see with
the unaided eye (6th magnitude or fainter is desirable).
Cooling the Telescope
All optical instruments need time to reach "thermal equilibri-
Collimated
um". The bigger the instrument and the larger the temperature
change, the more time is needed. Allow at least 30 minutes
for your telescope to cool to the temperature outdoors. In very
cold climates (below freezing), it is essential to store the tele-
scope as cold as possible. If it has to adjust to more than a
40° temperature change, allow at least one hour.
Let Your Eyes Dark-Adapt
Don't expect to go from a lighted house into the darkness of
the outdoors at night and immediately see faint nebulas,
galaxies, and star clusters—or even very many stars, for that
matter.Your eyes take about 30 minutes to reach perhaps 80%
of their full dark-adapted sensitivity. As your eyes become
dark-adapted, more stars will glimmer into view and you'll be
able to see fainter details in objects you view in your telescope.
To see what you're doing in the darkness, use a red-filtered
flashlight rather than a white light. Red light does not spoil
your eyes' dark adaptation like white light does. A flashlight
with a red LED light is ideal, or you can cover the front of a
regular incandescent flashlight with red cellophane or paper.
Beware, too, that nearby porch and streetlights and car head-
lights will ruin your night vision.
Calculating the Magnification
It is desirable to have a range of eyepieces of different focal
lengths, to allow viewing over a range of magnifications. To
calculate the magnification, or power, of a telescope, simply
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